Remove unused Document::getFocusableNodes

This function was also deleted in WebKit project.
Please refer to <https://trac.webkit.org/changeset/150699> for more details.

Review URL: https://chromiumcodereview.appspot.com/19269008

git-svn-id: svn://svn.chromium.org/blink/trunk@154300 bbb929c8-8fbe-4397-9dbb-9b2b20218538